1. Field of the Invention
This invention relates to a family of new photoinitiators, to new compositions comprising a free-radical addition polymerizable or crosslinkable compound and a photoinitiator system for the compound which includes a xanthene dye derivative, and to a method for producing three dimensional and other bodies by photopolymerization of the fluid monomer, and, more particularly, to such a method in which a laser is used to provide activating radiation for the photoinitiator system for the monomer. The invention also relates to a method for producing a three-dimensional model of an internal organ of a human or of a lower animal.
2. The Prior Art
Broadly speaking, a body of a composition comprising a photopolymerizable monomer, usually an acrylate or an acrylate derivative, and a photoinitiator system therefor can be caused to undergo photopolymerization by subjecting it to activating radiation of a suitable wave length. The photoinitiator system includes a compound, called a "photoinitiator", and, usually, an "accelerator". Activating radiation causes the photoinitiator to undergo a reaction, producing an intermediate which includes a free radical chain and, as a consequence, causes the monomer to polymerize. The accelerator, frequently a tertiary amine or a borate which, among other things, acts to ameliorate the effect of oxygen, which is to terminate the free radical chains that are produced by the reaction of the photoinitiator and, as a consequence, to quench the free radical polymerization.
Most photopolymerizations are carried out to polymerize monomers in relatively thin films, for example, to "dry" printing ink after it has been applied to paper, or to cure solventless varnishes, adhesives and photoresists. Indeed, U.S. Pat. No. 4,575,330 granted Mar. 11, 1986 to Hull, "The 3-D Patent", discloses a method for producing three dimensional articles by a process which involves a plurality of photopolymerizations, one to form each of a large number of thin films which ultimately make up the desired article.
Most photoinitiators that have been used commercially are activated by ultraviolet radiation, i.e., radiation having a wavelength shorter than 400 nm, and usually from mercury lamps, for example mercury resonance lamps with output at 313 nm and 364 nm. An ultraviolet laser is used in the process disclosed by The 3-D Patent, but such a laser, at the present time, has some limitations. It is not reliable, has a short lifetime, is expensive and has a high power requirement. Recently, photoinitiators that are activated by visible light have also been used, notably in a colored copying process that is available under the trade designation "CYCOLOR". Benzoin ethers and related acetals, Hammond's initiators, diaryliodonium and triaryl sulfonium salts and various photosensitive peroxides and peresters are examples of known photoinitiators that are activated by ultraviolet radiation (see, for example, U.S. Pat. No. 4,561,951, "Neckers", granted Dec. 31, 1985 to one of the present inventors). Cyanine dye, borate- so-called(+,-) ion pair initiators, xanthene dyes, and onium salt (-,+) ion pair photoinitiators are examples of known photoinitiators that are activated by radiation in the visible spectrum. The properties of polymers produced when these photoinitiators are used, however, are inferior to those of polymers produced when the photoinitiators are activated by ultraviolet radiation.